This invention relates to helical resonators and filter circuits employing helical resonators.
In some frequency ranges such as an extended range around 100 MHz, lumped-parameter filters may be objectionably lossy and perhaps too small to fabricate precisely for a particular application; yet, at the same time, it may not be desired to construct filters of waveguide-type configurations because they may be too bulky for the particular application or may not be otherwise compatible with the application environment intended.
The helical resonator, sometimes called a coaxial resonator with helical inner conductor, was developed to provide a solution to the High Q resonator problem in the frequency range in which both the pure lumped-parameter approach and the pure waveguide or distributed-parameter approach were for various reasons found to be awkward. It represents an intermediate type of solution having some aspects of a lumped parameter circuit and some aspects of a distributed parameter or guided-wave type of circuit. It has been found that helical resonators offer a practical solution to the need for filters of small percentage bandwidth, typically less than 2%, in the range from 30 MHz to 500 MHz.
A conventional helical resonator employs a single conducting helix mounted within a suitably proportioned outer conductor or conducting shield. One end of the helix is connected to the shield. The anti-resonant frequency is determined by the inductance and distributed capacitance existing in this coaxial cavity. Usually, additional reactance, such as trimming capacitance, is introduced for tuning purposes. Additional fundamentals on the techanical aspects of helical resonators may be found in the book by A. I. Zverev, Handbook of Filter Synthesis, Wiley (1967) pages 499-507.
When a number of these helical resonators are cascaded to form a coupled-resonator bandpass filter, it is found that additional passbands occur in the vicinity of odd multiples of the fundamental anti-resonant frequency or center frequency of the desired passband. These undesired passbands are so pronounced in the simple cascaded resonator configuration that special treatment is required when continuous stopbands are needed. Furthermore, since the coupling between resonators is usually achieved by the use of side wall apertures that reduce the resonator Q it is desirable to be able to minimize the number of such coupling apertures in a cascaded filter. In other words, coupled-resonator filters having a desired passband and continuous adjacent stopbands are desired, but with a superior resonator Q.
More generally, the helical resonator filter art is a relatively new art in which a greater variety of circuit components and design techniques would be desirable. Indeed, modified components could facilitate advanced design techniques.